Fluidized catalytic cracking (FCC) processes have been used extensively in the conversion of high boiling portions of crude oils such as gas oil and heavier components customarily referred to use as residual oils, reduced crude oils, atmospheric tower bottoms, topped crudes, vacuum resids, and the like, to produce useful products such as gasoline, fuel oils, light olefins and other blending stocks. The processing of such heavy hydrocarbon feedstocks which comprise very refractory components typically requires severe operating conditions including high temperatures which have presented problems with plant materials of construction, catalyst impairment and increased catalyst coking.
At present, there are several processes available for fluid catalytic cracking of such heavy feedstocks. A particularly successful approach which significantly diminishes such problems as mentioned above is described, for example, in U.S. Pat. Nos. 4,664,778; 4,601,814; 4,336,160; 4,332,674; and 4,331,533. In such processes residual oils or vapors thereof are contacted with hot finely-divided solid catalyst particles in a fluidized solid state in a reactor section, e.g. an elongated riser reactor, to produce cracked products comprising lower molecular weight hydrocarbons typically used in motor gasolines and distillate fuels. A catalyst regeneration section is connected by conduits to the reactor section, through which circulation of the catalyst is maintained to regenerate the catalyst on a continuous basis.
The regeneration section comprises two separate relatively lower and higher temperature catalyst regeneration zones which minimize the severity of catalyst regeneration. Hydrocarbonaceous deposits (coke) formed on the catalyst surface, after volatile hydrocarbons are separated therefrom, are initially combusted in a first catalyst regenerator zone in the presence of a restricted amount of oxygen-containing gas, e.g., air, at relatively mild temperatures sufficient to selectively burn most of the hydrogen component present in the coke deposits and some of the hydrocarbonaceous component to form a partially regenerated catalyst and a first regeneration zone flue gas effluent rich in CO. This relatively mild first regeneration serves to limit localized catalyst hot spots in the presence of steam formed during the hydrogen combustion such that the formed steam will not substantially reduce catalyst activity.
The partially regenerated catalyst now substantially free of hydrogen in the remaining coke deposits thereon which is recovered from the first regeneration zone is then passed to a second relatively higher temperature regeneration zone designed to minimize catalyst inventory and residence time at higher temperature while promoting a carbon combustion rate to achieve a recycled catalyst with significantly reduced coke content. This operation permits higher regeneration temperatures to be employed with a lower catalyst deactivation rate than is possible in single stage catalyst regenerators. In the second regeneration zone, remaining coke deposits are substantially completely burned to CO.sub.2 at elevated temperatures to form hot regenerated catalyst and a hot CO.sub.2 -rich second regeneration zone flue gas stream, useful, for example, in generating process steam.
In typical operation, regeneration of catalyst particles by combustion of hydrocarbonaceous deposits thereon in the regeneration zones is effected by maintaining the particles in a fluidized condition in the presence of the combustion gas, e.g. air. The combustion air thus additionally acts as a fluidizing gas by passing upwards through the regeneration zones at a rate sufficient to maintain the particles in a fluidized bed, i.e., in a turbulent state with quasi-liquid properties. Some fluidizing air is also employed as a transfer medium to circulate the catalyst particles continuously through the regenerator and reactor sections.
Such fluidized catalytic cracking processes, especially those employing two or more catalyst regeneration zones as described above, can thus require large volumes of compressed fluidizing/combustion and transfer gas, e.g. compressed air, with corresponding extensive investment in facilities required to operate the air compressors. Power facilities and motive power supply have therefore become some of the major expenses of fluidized catalytic cracking of residual oils.
Flue gases which emerge from the first and second regeneration zones in such processes as described above represent a large energy potential which can be utilized to supply at least part of the power used in the system for compressed air requirements. For example, the flue gases, usually at high temperature and elevated pressure, can be passed to respective tertiary separators to remove particle fines or solids, and then directed to respective expansion turbines to supply power to an air compressor serving as a source of compressed air for the regeneration process. Further, the combustion of CO to CO.sub.2 in the CO-rich effluent flue gas from the first regeneration zone is highly exothermic and liberates large quantities of heat energy, and thus is also an attractive source of process energy from the regenerators.
The combustion of CO-containing flue gas is usually performed under controlled conditions downstream from a catalyst regenerator in a separate CO-boiler or combustion device enriched with air and continuously fed with CO-containing flue gas. The CO-boiler can be equipped to accept at least one other fuel which is used in start-up, or more commonly to supplement the fuel value of the flue gas, or to provide a process fuel when the catalytic cracking apparatus itself is shut down. Such processes are well known. For example, U.S. Pat. Nos. 3,702,308 and 3,401,124 disclose supplying regenerator flue gas to an exhaust gas turbine used to drive a generator, then burning the combustible part of CO contained in the flue gas in a catalytic CO-boiler or in the presence of air and supplementary fuel to recover maximum sensible and combustion heat from the flue gas for use elsewhere in the process. Other examples are described in U.S. Pat. No. 2,753,925 wherein the released heat energy from CO-containing flue gas combustion is employed in the generation of high pressure steam. U.S. Pat. Nos. 3,137,133 and 3,012,962 describe flue gases which are expanded in turbines to produce shaft work. A further example is described in U.S. Pat. No. 3,247,129 wherein exit gases from a catalyst regenerator are led to a boiler in which they are burnt under pressure, supplemental fuel and air being added, after which the combustion gases are discharged from the boiler and expanded in a gas turbine/compressor unit which supplies air for the regenerator and the combustion air for the boiler.
At present, it would therefore be desirable to combine the CO and CO.sub.2 -rich effluent flue gases from the respective first and second catalyst regeneration zones such that he combined stream can be passed to a single tertiary separator to remove entrained catalyst fines and/or solids, and then expanded in one downstream expansion turbine-compressor unit to supply at least a part of the compressed air required for the regenerators, therein providing significant savings in both equipment and operating costs. It would further be desirable to operate an expansion turbine-compressor unit fed by the combined flue gases which compresses sufficient gas to meet substantially all fluidizing/combustion gas demands, whereby the process can be substantially self-powering.
However, combination of the CO-rich effluent flue gas from the first regeneration zone with flue gas effluent from the second regeneration zone which can contain quantities of oxygen, at high temperatures and pressures may cause combustion or microburning of the combined streams leading to temperatures exceeding the metallurgical limits of process materials. Further, typical heavy hydrocarbon feedstocks cracked in the process may contain many impurities including vanadium, chromium, nickel and other metals found in Groups IB-VIII, inclusive, of the periodic table which can dope or otherwise bind to at least a portion of the catalyst particles and function as combustion promoters during the catalyst regeneration stage. Catalyst dust or particles which exit the regenerators in the flue gas can thus greatly facilitate combustion after combination of the effluent flue gas streams from the first and second regenerator zones.